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          <h1 class="post-title" itemprop="name headline">网络编程之TCP状态</h1>
        

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        <p>网络编程之TCP状态</p>
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<hr>
<h1 id="TCP状态"><a href="#TCP状态" class="headerlink" title="TCP状态"></a>TCP状态</h1><p><img src="/2019/07/13/networksocket4/tcp_connect.png" alt></p>
<p>​                                            TCP三次握手</p>
<p><img src="/2019/07/13/networksocket4/tcp_close.png" alt></p>
<p>​                                        TCP四次挥手</p>
<hr>
<h1 id="SIGPIPE"><a href="#SIGPIPE" class="headerlink" title="SIGPIPE"></a>SIGPIPE</h1><p>往一个已经接收FIN的套接字写时允许的，接收到FIN仅仅代表对方不再发送数据。<br>在收到RST段之后，如果再调用write就会产生SIGPIPE信号，对于这个信号的处理的处理我们通常忽略即可。<br>signal(SIGPIPE, SIG_IGN);</p>
<hr>
<h1 id="TIME-WAIT状态"><a href="#TIME-WAIT状态" class="headerlink" title="TIME_WAIT状态"></a>TIME_WAIT状态</h1><p>TIME_WAIT状态对大并发服务器的影响，应尽可能在服务器避免出现TIME_WAIT状态，如果服务器端主动断开连接，服务端就会进入TIME_WAIT状态，协议设计上，应该让客户端主动断开连接，这样就把TIME_WAIT状态分散到大量的客户端。如果客户端不活跃了，一些客户端不断开连接，这样子就会占用服务器端的连接资源。服务器端也有有个机制踢掉不活跃的连接。<br>TIME_WAIT状态存在原因如下：</p>
<ul>
<li>保证可靠地终止TCP连接<br>处于TIME_WAIT状态的发送端会向目的端发送ACK，如果此时ACK丢失，目的端会超时重传FIN报文段，目的端收到重传的报文段最少需要2<em>MSL，所以发送端会等待2</em>MSL时间。</li>
<li>保证迟到的TCP报文段有足够的时间识别被丢弃</li>
</ul>
<hr>
<h1 id="EMFILE错误"><a href="#EMFILE错误" class="headerlink" title="EMFILE错误"></a>EMFILE错误</h1><ul>
<li>调高进程文件描述符数目</li>
<li>死等</li>
<li>退出程序</li>
<li>关闭监听套接字。那什么时候重新打开呢？</li>
<li>如果是epoll模型，可以改用edge trigger。问题是如果漏掉了一次accept(2)，程序再也不会收到新连接。</li>
<li>准备一个空闲的文件描述符。遇到这种情况，先关闭这个空闲文件，获得一个文件描述符名额;再accept拿到socket连接的文件描述符；随后立刻close，这样就优雅地断开了与客户端的连接；最后重新打开空闲文件，把“坑”填上，以备再次出现这种情况时使用。</li>
</ul>
<hr>
<h1 id="RST报文段"><a href="#RST报文段" class="headerlink" title="RST报文段"></a>RST报文段</h1><p>产生复位报文段RST的情况：</p>
<ul>
<li>访问不存在的端口或者处于TIME_WAIT状态的端口</li>
<li>异常终止连接</li>
<li>处理半打开连接</li>
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#TCP状态"><span class="nav-number">1.</span> <span class="nav-text">TCP状态</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#SIGPIPE"><span class="nav-number">2.</span> <span class="nav-text">SIGPIPE</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#TIME-WAIT状态"><span class="nav-number">3.</span> <span class="nav-text">TIME_WAIT状态</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#EMFILE错误"><span class="nav-number">4.</span> <span class="nav-text">EMFILE错误</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#RST报文段"><span class="nav-number">5.</span> <span class="nav-text">RST报文段</span></a></li></ol></div>
            

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